Such a material testing system has a configuration in which, for example, a pair of screw rods is supported on a table rotatably in synchronization with each other, and both end parts of a crosshead are supported by the screw rods through nuts. Also, by using rotation of a motor to rotate the pair of screw rods in synchronization with each other, the crosshead is moved along the pair of screw rods. The crosshead and the table are respectively connected with grippers. The material testing system is configured to, in a state where both ends of a test piece are gripped by the pair of grippers, move the crosshead to thereby apply a load to the test piece.
In such a material testing system, as the motor for rotating the screw rods, a servomotor is typically used. For example, as described in JPA 2002-365188 as Patent literature 1, among test force acting on a test piece, elongation of the test piece, and the like, a transient detection value of a physical quantity selected as a control variable is set as a target value to perform feedback, and thereby a motor is controlled. That is, in the case where the test force is selected as the control variable, a calculation such as PID is performed on a deviation between a detection value (current value) of the test force and the target value, and a value corresponding to a result of the calculation is given as a rotational angular speed command to a servo amplifier, which supplies a driving signal to the driving motor, to thereby control the motor. More specifically, a value obtained by multiplying the deviation by a constant (proportional gain), a value obtained by multiplying time derivative of the deviation by a constant (derivative gains), and a value obtained by multiplying time integration by a constant (integral gain) are combined to determine the magnitude of the rotational angular speed command.
In a material testing system using such a motor as a driving source, unless the above-described proportional gain, derivative gain, and integral gain are adequately set, accurate material testing that correctly follows a target value cannot be performed. However, the real situation is that the gains are set by trial and error. For this reason, there are problems that, in particular, when material testing is performed on a new type of a test piece, it takes long time to set the gains, and also an operator should be skillful.
JPA 2009-14499 as Patent literature 2 discloses a material testing system that uses a motor as a driving source of a loading mechanism for applying a load to a test piece, and is provided with a control mechanism that successively gives a rotational angular speed command for the motor to a servo amplifier driving the motor and thereby controls operation of the load mechanism. In the material testing system, test force that serves as a control variable and acts on the test piece, or test piece elongation as a control variable is first detected as a detection value. Then, according to a deviation between the detection value and a target value, the magnitude of a rotational angular speed, which is to be supplied to the servo amplifier as the command, is computed. After that, a ratio of the detection value of the control variable to a displacement of the motor is successively computed, and a value proportional to a value obtained by dividing the deviation by the computed ratio is set as the magnitude of the rotational angular speed, which is to be supplied to the servo amplifier as the command.
According to the material testing system described in JPA 2009-14499, without requiring skill for setting of control gain, or doing trial-and-error work for the setting for a long time, accurate material testing can be constantly performed.